The art of air plasma spraying (APS) involves applying material to a surface in an ambient atmosphere. One known limitation of conventional plasma spraying techniques, is that they are limited to using large feedstock powders (10-100 μm). Consequently techniques for suspending finer powders (e.g. nano-scale dimension powders) in a liquid carrier have been developed to permit feeding and spraying of much finer particles. Unlike to the very small individual particles, the liquid suspension droplets possess sufficient momentum to enter the plasma stream. The plasma-liquid interaction atomizes the nano-sized powder suspension into a fine mist and evaporates the liquid suspension medium inside the flame, thereby concentrating the solid content into micron-sized or even smaller particles. At impact on the substrate, these particles form thinner lamellar than in conventional plasma spraying. Due to the finer lamellar structure, the formation of thin (50-10 μm or less) thermal spray coatings is possible. Nanostructured coatings can be formed with this technique, as rapid solidification of the small impinging droplets restricts their grain growth. As is well known to those familiar with the art, certain mechanical and physical properties of materials exhibit remarkable improvements as their grain size is reduced to the nanometer range.
In the art of suspension thermal spray coating, it is known to use a DC torch at atmospheric pressure. In such embodiments a liquid feed system is typically provided to inject droplets radially into a plasma flame of the torch. To ensure effective heat and momentum transfer from the plasma to the droplets of the suspension, high droplet jet velocities, and precise injection location and angle with respect to the central part of the plasma are critical. Experiments have revealed that the characteristics and properties of the resulting coatings are highly sensitive to these injection conditions. At the same time, the feed rate of the suspension is limited by the finite thermal loading capacity of the plasma, which not only has to melt the particles but also evaporate the entire liquid carrier. Generating high droplet velocities at a suitable feed rate requires small injection orifices, which are prone to obstructions by the solid content of the suspension. Injection instabilities, which frequently occur during partial and temporary obstructions, can divert the jet, leading to disruptions in the spray, and consequent loss of process efficiency. Complete obstructions terminate the spray process prematurely.
To avoid these problems research has been directed towards atomization of the liquid carrier, and to precursor technologies which do not use suspension delivery of the particles. With radial injection of an atomized liquid jet the heating and dispersion of the injected droplets is strongly dependent on, among other variables, the trajectory and size of the atomized droplets and, ultimately, the size distribution in the atomized plume. It will be evident to those skilled in the art that any droplets having too much or too little momentum will not be entrained in the plume, leaving a narrow band of droplet size and velocities suitable for effective delivery. Heat and momentum transfer to the carrier within the plasma is far more sensitive to droplet size and velocity and it is difficult to produce droplets in a narrow distribution of sizes and velocities. For example, U.S. Pat. No. 6,579,573 B3 to Strutt et al. discloses a method whereby nanoparticle liquid suspensions are used in conventional thermal spray deposition for the fabrication of high-quality nanostructured coatings, and the liquid is gas atomized prior to radial injection into the plasma flame.
For other reasons axial injection of feed in a plasma spray system has been developed. For example, U.S. Pat. No. 4,982,067 to Marantz et al relates to an apparatus to eliminate the long-standing problems with radial feed spray apparatus by designing a true axial feed in a plasma spay system. While most of this disclosure is to using particles as the feed, the patent also states that, “alternatively the feedstock may be liquid form, such as a solution, a slurry of a sol-gel fluid, such that the liquid carrier will be vaporized or reacted off, leaving a solid material to be deposited”.
U.S. Pat. No. 5,609,921 to Gitzhofer discloses a suspension plasma spray where the material is supplied to the plasma discharge in the form of a suspension. The suspension is brought into the plasma discharge by an atomizing probe using a pressurized gas to shear the suspension, and thus atomize it into a stream of fine droplets. The atomizing probe also includes a cylindrical suspension injection tube. Between the injection tube and an inner tube (which encases the injection tube) is defined an annular chamber supplied with an atomizing gas. While most of the disclosure refers to a radio frequency induction torch spray system in a controlled pressure reactor chamber, the patent also states that the RF plasma torch could be replaced by a DC plasma torch.
U.S. Pat. No. 6,491,967 to Corderman relates to a plasma spray high throughput screening method and system for fabrication of thermal barrier coating. FIG. 3 of this patent shows a schematic of a gas atomizing injector for the liquid feedstock, where the liquid reactant is introduced in the center tube of two concentric tubes. FIG. 4 of this patent shows a version of the apparatus configuration for suitable DC APS torches, where this injector is installed internal to the torch. This patent essentially deals with solution precursor feeds, in which reactants (deposition material) are dissolved.
A publication in J. Am. Ceram. Soc. 81 [1] 212-28 (1998) to Kathikeyan et al. relates to nanomaterial deposits formed by dc plasma spraying of liquid precursor feedstocks. An internal gas-atomizer, consisting of multiple concentric tubes where the liquid is supplied in a central tube, injects the liquid feedstock axially into the center of four converging plasma flames.
There remains a need for a solution to the problem of obstructions of a small injection orifice, to deliver at a low flow rate and sufficient velocity, while reducing obstructions for a suspension feedstock delivery system. Furthermore an Air Plasma Spraying (APS) system is desired that permits suspension feedstock to be controlled and delivered with reduced sensitivity of the spray process on the injection conditions to enable production of nanostructured coatings.